Optical analysis of fluids plays an important role in fields such as chemistry, microbiology and biochemistry. These fluids may include liquids or gases and may provide reagents, solvents, reactants, or rinses to chemical or biological processes. While various microfluidic methods and devices, such as microfluidic assays, can provide inexpensive, sensitive and accurate analytical platforms, carrying out accurate optical measurements (e.g., absorbance or transmission) on a microfluidic system can be challenging. Optical measurements of microchannels may require, for example, time-consuming alignment procedures. In addition, optical noise produced by light incident upon areas outside the channels may degrade the quality of the detected signal through the channels. Accordingly, advances in the field that could reduce costs, simplify use, and/or improve optical detection in microfluidic systems would be beneficial.